I've found the "Evaluation of Expressions" doc page to be super useful: https://reference.wolfram.com/language/tutorial/EvaluationOf...

Read through it once to get the gist, then go back and read it again. The second time through, _play_ with each example. Try them out, see how they behave, attempt to apply other things you've learned to each. In no time at all, you'll have the basics down and then will be able to make heads or tails of the other doc pages much more quickly.